1. Field of the Invention
The present invention relates to a method of identifying under-performing oil wells in a large field with a long production history.
2. Description of the Prior Art
Initial hydrocarbon production from subterranean reservoirs is generally referred to as “primary” production. During primary production, only a fraction of the hydrocarbon in the reservoir is recovered. Thereafter, additional hydrocarbons may be recovered by employing enhanced hydrocarbon recovery techniques e.g. by injecting fluids such as water, steam, nitrogen, CO2 or natural gas into the reservoir and such subsequent production is generally referred to as “secondary” or “tertiary” production. Enhanced recovery techniques generally depend on the injected fluid to displace the hydrocarbon from its in-situ location and direct it towards a producing well from which it can be recovered. Because of the substantial economic cost required to develop and implement enhanced recovery techniques, it is critically important for a reservoir engineer to characterize the storage and flow capacity of a hydrocarbon bearing reservoir.
Experience in the petroleum industry has indicated that reservoir storage and flow parameters obtained from geological, geophysical and petrophysical data can be used to develop a model of the reservoir and thereafter the model can be inputted into a numerical reservoir simulator to obtain predictions of reservoir response or performance during enhanced hydrocarbon recovery. The goal of such numerical reservoir simulators is to predict reservoir performance in more detail and with more accuracy than is possible with simple extrapolation techniques.
An analytical technique for estimating well drainage areas in well reservoirs is disclosed by J. S. Anderson in the paper entitled “Pressure Mapping as an Aid to Understanding Reservoir Drainage,” SPE 22962 (1991). That technique is based on calculating reservoir pressure throughout the field in question and producing pressure maps over the field. According to Anderson, streamlines tracing the path of fluid toward the well can be plotted and drainage areas can be discerned from the pressure mapping. Anderson discloses a mathematical/analytical technique which is believed to be suitable for use with simple reservoirs, e.g., those having homogeneous properties and/or simple geometries.
No method has heretofore been developed which is based on numerical methods which can handle more geologically realistic reservoir descriptions, which uses the drainage area concept specifically to determine the recovery efficiency of the wells and how this evolves over field life, and which uses the concept of recovery efficiency on a well-by-well basis to identify inhibited wells or wells with erroneous (i.e., systematic under-reported/under-allocated) production figures. These results have been achieved by the method of the present invention.